1. Field of the Invention
The present invention relates generally to an assembly for information processing and in particular to an assembly for thermal memory cells and chips and to an assembly for thermally-assisted magnetic information processing as well as thermal protrusion control. The assemblies include a heater and a controller for controlling the heater.
2. Description of the Related Art
One of the key challenges in any form of thermal and/or thermally-assisted information processing and control and/or thermally-induced protrusion control is to optimize the performance of the energy source (e.g., heater). For example, in a thermal memory cell/chip a heater is used to write and erase on a storage media. In some cases, for the purpose of reading, the same heater is utilized to measure the thermal impedance of the storage media. In other schemes of phase-change recording as described in the above-referenced related U. S. patent application Ser. Nos. 09/774,851, 09/774,943, 09/733,323, and 09/773,346, a heater in a head-like structure is used to realize thermal reading, writing and erasing.
As discussed, for example, in U.S. Pat. Nos. 6,233,206 and 6,493,183, thermally-assisted magnetic recording involves heating the storage media by transferring energy (i.e., heat) to the media from an energy source (e.g., a heater), which may be embedded in a recording head. The resulting temperature rise in the magnetic medium reduces its coercivity, which allows the use of harder, and therefore, higher density capable recording media. Without heating, the available magnetic field would be not sufficient to record the magnetic information patterns. More specifically, for a about 20% writeability improvement on a conventional longitudinal recording media (e.g., a coercivity of about 20 Oe/K) the media has to be heated up to about 80° C. by the heater element. Depending on various details such as the size of the heater (e.g., heating element) size, spin speed, media thermal and substrate thermal properties, heat flux between the recording head and disk etc., the heater has to be heated to about 300° C. or even more. As a final example, heaters in (magnetic) recording heads can be used to induce protrusion, which reduces a fly height (e.g., distance) between heater and recording media.
The most economic way of supplying energy to the heater includes Joule heating, in which electrical power is dissipated in the heater, causing an increase in the temperature of the heater (e.g., heating element). In order to simplify the discussion of the present invention, in the following it will be assumed that the resulting temperatures are linearly dependent on the dissipated power in the heater.
However, Joule heating is accompanied by several challenges because the heater has to last and maintain its properties for an extended period of time at these high temperatures (e.g. >1000 hours). Typical failure mechanisms besides dielectric breakdown and ESD, include oxidation, electro-migration, thermal breakdown and thermal stress.
In particular, electro-migration can be a real issue since the currents supplied to a small resistor heater are considerably close to current density limits. Specifically and as an example, for most moderate thermally-assisted magnetic recording with a heater cross-sectional area of 0.5×0.03 μm2, heater length of 5 μm and a heater material of beta-Ta (2×10−6 Ωm) finite element modeling shows that 5 to 6 mA and 3 to 4 V are needed to heat the disk to 80° C., which corresponds to a current density of about 5×107A/cm2. Experimentally, it can be shown that at higher temperatures these current densities alter the resistance of the Ta-heater by about 0.5% /hour after 2 hours at 500° C.
The heater can often be operated in different modes. The two obvious modes include constant current and constant voltage DC-power supplies. However, both of these methods are only usable if the heater resistance is not altered significantly. For example, if a constant DC current is supplied to the heater and the heater resistance increases slowly, the dissipated power in the heater and consequently the temperature increases as well. Therefore, such a mode can very easily cause an “avalanche effect”, which results into the failure of the heater (e.g., heating element).
On the other hand, if a constant DC voltage is supplied to the heater, the increased resistance will cause the current to decrease, which results into less power dissipated and thus, a lower temperature. Since the temperature of the heater has to be maintained and controlled precisely, the constant current (as well as the constant voltage) mode is not acceptable as long as the heater resistance changes. For example, experiments on the aforementioned Ta-heaters have shown that at a constant voltage, which corresponds to a temperature of 500° C., the resistance drops more than 20% in 12 hours, which means that at a constant voltage the temperature in the heater has come down by almost 100 K.
In summary, a good assembly design has two objectives: a) minimize heater resistance changes and b) control the power dissipated in the heater and thus control the temperature of the heater. However, conventional controllers and assemblies do not effectively address these objectives.